Radionuclides are employed as radioactive labels, or tracers, by incorporating them into molecules to produce a radio-labeled probe. The probes are introduced into a patient (source) and become involved in biological processes, such as blood flow, fatty acid, glucose metabolism, and protein synthesis. The probes can also be formulated to accumulate differently in targeted tissue (a target), such as an organ of interest, relative to other tissue, such as due to elevated (or diminished) rates of glucose metabolism in diseased cells compared to normal tissue. Alternatively, targeting can be achieved by using probes having engineered antibodies or anti-body fragments that bind to receptors present in target tissue, such as, for example the -Her2-Neu receptor over-expressed by breast cancer cells. A widely used -imaging probe, 18-Fluoro-deoxyl-glucose (FDG), has been used, for example, in vivo for imaging of cancer, neuro-degenerative disease and cardiovascular disease. Similarly, Alzheimer's disease can be detected by using probes that target beta amyloid, which tends to accumulate in a diseased brain.
As the radionuclides decay, they may emit either gamma rays or positrons. In the case of positron emission, the positrons travel a very short distance before they encounter an electron, and when this occurs, they are annihilated and converted into two photons, or gamma rays. By measuring the number of pairs of photons emitted in the target relative to the other tissue, organ characteristics or irregularities can be studied. In the case of direct gamma ray emission, by measuring the number of gamma rays emitted in the target relative to the other tissue, organ characteristics or irregularities can be studied
Currently, nuclear medical imaging includes Planar Gamma Camera Imaging, Single-Photon Emission Computed Tomography (SPECT), Positron Emission Tomography (PET) and Multiple Emission Tomography (MET). Planar Gamma Camera Imaging is performed by a gamma camera, such as an Anger camera, having a collimator typically made of lead or tungsten having a plurality of channels, and a scintillation crystal placed under the collimator. Gamma rays traveling along a path that coincides with a path defined by any of the channels pass through the channel unabsorbed and interact with the crystal for producing a light signal. An array of light sensors, such as photomultiplier tubes (PMTs), is provided behind the crystal to detect the light and generate an intensity signal indicative of the amount of light detected at each sensor. A planar image is constructed in accordance with the intensity signal and the direction from which the associated gamma signal came from. SPECT includes generation of a 3-dimensional image by generating and reconstructing a plurality of planar images from different angles using a tomographic process.
In a PET scanner, characteristics of two aspects of the annihilation of positrons are of particular interest; each gamma ray has an energy of 511 keV, and the two gamma rays are directed in nearly opposite directions. The PET scanner is typically cylindrical, and includes a collimator for rejecting scatter events, and a detector ring assembly composed of rings of detectors which encircle the patient, and which convert the energy of each 511 keV photon into a flash of light that is sensed, such as by a PMT. Coincidence detection circuits connect to the detectors and record only those photons which are detected simultaneously by two detectors located on opposite sides of the patient. The number of such simultaneous events (coincidence events) indicates the number of positron annihilations that occurred along a line joining the two opposing detectors. Within a few minutes hundreds of millions of coincidence events may be recorded indicative of the number of annihilations along lines joining pairs of detectors in the detector ring. These numbers are employed to reconstruct an image using well-known computed tomography techniques. MET utilizes two-different crystals positioned in a sandwich-like construction for allowing simultaneous use of gamma emitting and positron emitting radio-labeled probes.
There are many factors during the imaging process, which affect the degree of qualitative and quantitative accuracy of the image produced. Such factors include, for example, selection, configuration, placement and/or function of components of the nuclear scanner, including collimator dimensions, source-to-detector distance, resolution of the gamma camera; timing and duration of image capture; image reconstruction techniques; and composition of the source. However, due to expense of the radionuclides and imaging probe, the ill-effects of exposure of the patient to the radionuclides, expense of test administration, and difficulty in separating out single factors of the entire imaging process in an experimental situation, it is generally impractical to empirically study or make adjustments to the imaging process. Evaluation of probes has commonly been performed through animal experiments and subsequent human trials; however the process is inefficient, slow and expensive.
Accordingly, tools have been developed, such as described by J. C. Yanch et al. in “Physically Realistic Monte Carlo Simulation of Source, Collimator and Tomographic Data Acquisition For Emission Computed Tomography”, Phys. Med. Biol., Volume 37, No. 4, 1992, pp 853-870, for simulation of nuclear imaging, in which a user may input information detailing configuration of the nuclear imaging scanner including the number of tomographic views; the source, including geometries within, and “the 3D distribution of the isotope and various attenuating materials . . . distributed in any spatial configuration throughout an organ inside the human body or head”; and image reconstruction parameters, including energy windows and energy sampling functions.
However, the image generated by simulated nuclear imaging is limited to providing spatial information at a specific point in time corresponding to the input time of the data associated with the simulated isotope distribution. However, due to the nature of isotopes, the real isotope distribution varies over time.
Accordingly, there is a need for a system and method for simulated nuclear medicine imaging which accounts for temporal changes in isotope distribution.